The present invention relates to a method of producing a calibration wafer having at least one predetermined optical characteristic, in particular a predetermined emissivity.
Computer chips, as well as other electronic components, are manufactured on round, disk-shaped semiconductor bodies, so-called wafers. During the manufacture, the wafers are subjected to various operations and processes. With some processes, it is necessary that the wafers follow a prescribed temperature curve. For example, coating processes of the wafers are frequently effected in that the wafers are subjected to a prescribed temperature curve during which they are disposed in a prescribed process atmosphere. It is also known to thermally treat wafers in a vacuum, and in particular as a post treatment process to preceding treatment steps. For example, a thermal treatment can heal damages in the crystalline lattice structure of the wafer that result as a consequence of an ion implantation.
In recent times, so-called RTP units (Rapid Thermal Processing) are used ever more frequently for the thermal treatment of the wafers, with such units also being designated as rapid heating units. Such units enable a rapid thermal treatment of the wafers under prescribed process conditions at temperatures of up to 1200° C. What is special about these units is not only the high treatment temperature, but also that heating rates of 200° C./s and more can be achieved. Due to these high heating rates, which ensure a high throughput of the RTP units, they have a significant economical advantage. Furthermore, high heating and cooling rates are in particular important for a successful treatment of very small structures, since a treatment at high temperatures is possible while at the same time the overall thermal budget of the thermal processing can be kept low.
However, the rapid heating-up rates also lead to a considerable loading of the wafers if they are not homogeneously treated over their entire surface. As a result, temperature differences can occur between different regions of the wafer, which can lead to a distortion of the wafer or the formation of crystalline structure imperfections or defects. A distortion or crystalline structure imperfection can destroy the structures or electrical contacts that are applied to the wafer, thus making the wafer unusable. Therefore, a homogeneous temperature distribution over the entire wafer is necessary. To achieve this, a permanent temperature measurement is effected during the thermal treatment for the control and regulation of the temperature curve or progress of the wafer. In this connection, a temperature measurement, in particular also at different points of the wafer, is effected in order to compensate for temperature inhomogeneities. A reliable temperature measurement is therefore a main feature of an RTP unit.
For the temperature measurement, pyrometry has proven to be particularly expedient, since it requires no contact with the wafer and furthermore does not adversely affect the actual heating of the wafer by sources of radiation. A temperature measurement in RTP units based on pyrometry, however, has basic problems due to an intensive radiation field within a processing chamber of the RTP unit. The radiation field, which is generally emitted from heating lamps, is customarily so great that it superimposes a temperature radiation that is emitted from the wafer and is to be measured by the pyrometer. This problem is intensified at low wafer temperatures since the wafers have a low emissivity at low temperatures. Due to the low emissivity of the wafers at low temperatures, however, the signal-to-background ratio is made even worse.
U.S. Pat. No. 5,154,512 discloses a method for measuring a wafer temperature in an RTP unit, according to which a periodic modulation is imparted to the heat radiation. This modulation enables a differentiation between the heat radiation emitted from the heat sources, and the heat radiation emitted from the wafer, since the modulation is not contained in the radiation of the wafer. In the known method, initially a measurement signal, which is composed of the wafer temperature radiation and a portion of the heat radiation reflected at the wafer surface, is measured with a first contact-free measuring instrument. A measuring signal of the heat radiation of the heat device is received by a second contact-free measuring instrument. In the known method it is presumed that the thermal mass of the wafer is so big that the wafer temperature cannot follow the imparted modulation of the heat radiation. Thus, it is possible to separate the non-modulated, weak wafer temperature radiation from the much greater, yet modulated, heat radiation metrologically. The wafer temperature can then be determined from the wafer temperature radiation.
With modern RTP units, the known method of the temperature measurement has been expanded and improved by a mathematical model. With this model, various radiation components of the unit, such as the occurrence of multiple reflections and others are taken into account. Therefore, the model contains a set of specific parameters with which geometrical and unit-specific factors are determined. The measurement signals measured in the measuring device are incorporated into this model, and the temperature of the wafer can be determined by means of the parameters of the model.
In this connection, in most cases a measurement of the parameters has turned out to be very difficult or, in practice, cannot be carried out. Therefore, it is necessary to have a starting calibration of the RTP unit at which values for the parameters of the model are determined by means of a calculation algorithm, with the parameters being as close to reality as possible. During the starting calibration, a plurality of calibration wafers having many different optical characteristics are measured at different temperatures, i.e. the temperature radiation emitted from the respective calibration wafers is measured.
At low temperatures, in particular temperatures below 600° C., however, the wafers have an increasing transparency for the heat radiation, which leads to a significant lowering of the emissivity and hence to a very low signal-to-background ratio. In practice, in particular metallic coated wafers are treated at low temperatures, with these wafers having much higher emissivity than do conventional calibration wafers. Therefore, in order to ensure a proper calibration even at low temperatures, special wafers having specific characteristics are necessary that have an emissivity that is similar to that of the wafers that are being treated at these low temperatures.
One possibility of making such a calibration wafer is to provide the calibration wafer with a metallic layer, and in particular similar to a wafer that is to be subsequently thermally treated. Due to the metallic layer, one achieves an adaptation of the emissivity of the calibration wafer to that of the wafer that is to be subsequently treated. However, this method has the drawback that the metallic layer can lead to an undesirable contamination of the unit. Furthermore, such metallic coatings are stable only in a limited temperature range, and therefore have only a limited use. If during the calibration process such wafers are heated to higher temperatures, the metal layer can peel off, which can lead to considerable contamination of the unit. Furthermore, the calibration wafer would be destroyed.
Therefore, for a good calibration it is important to make available calibration wafers which cover the ranges of temperature and emissivity that are relevant for the practice, i.e. that, at the respectively utilized temperature ranges, they have an emissivity that is similar to that of the wafers that are to be subsequently treated. At high temperatures, the wafers are optically opaque for the heat radiation, and therefore the emissivity at high temperatures can be altered merely over the reflectivity of the wafers if, for example, a suitable coating is selected. At low temperatures, where the wafer is essentially optically transparent for the heat radiation, an adjustment of the emissivity can be effected both via the reflectivity and the transmissivity.
It is therefore an object of the present invention to provide a calibration wafer that in a simple and economical manner has a predetermined optical characteristic, in particular a predetermined emissivity, that is adapted to the emissivity of the actually to be treated wafer, and with which there is no risk of metallic contamination of the unit.